Commercial and industrial storage tanks are widely used for storing a great variety of liquids. Some of these liquids are highly corrosive and/or are flammable. The service life of a storage tank will vary, depending upon environmental conditions, including the liquid being stored. Eventually, however, the tank will become corroded and develop leaks. This can result in a significant danger to the environment and health of nearby residents. For example, storage tanks are commonly used for storing gasoline at service stations. Gasoline, of course, is highly-flammable and is capable of posing a significant health and safety hazard if not properly contained. Federal as well as local regulations govern the structure of such storage tanks.
Heightened public awareness of the danger posed by storage tanks (particularly underground gasoline storage tanks) has led to additional governmental regulations. Recent proposed regulations will soon require most storage tanks to have secondary containnment means and possibly a fail safe design feature to guard against accidental soil, water, and air contamination. Secondary containment means must be capable of containing leaked liquid from the storage tank. Rigid double walled tanks made from sheet metal have been suggested as one alternative. While effective for containment purposes, such tanks, as presently available, are costly to build and difficult to install because of their weight. The tanks are built by basically forming two rigid tanks and placing one inside the other.
Tanks made from fiberglass reinforced resinous material are also extensively used. One common method of making these tanks comprises utilizing a reuseable cylindrical-shaped half-mold in the formation of the tank's body. Initially, layers of fiberglass followed by a resinous coating are applied to the mold or chopped fiberglass/resin streams are simultaneously directed onto the mold and subsequently cured. Sufficient applications of the fiberglass and resin are made until a wall thickness is obtained which has the desired strength. Next, cardboard rib forms, four to six inches wide, are placed completely around the covered mold at approximately sixteen inch intervals and fiberglass/resin applied. The cardboard forms result in a cylindrical-shaped tank with ribs. The purpose of the ribs is to add strength to the tank. The mold is finally removed. A cylindrical-shaped wall, including the support ribs are produced in this stage of the method. An end cap is either fabricated during the cylinder wall-making step or added after the mold is removed. The above steps are repeated to obtain a second half-tank. The two half-tanks are then joined together by appropriate sealing means. The formed tank is representative of a tank constructed by adding spaced support ribs to the outer surface of a cylindrical-shaped inner tank and then securing said ribs to the tank.
The single-walled tank above described is capable of being installed in the ground and, in fact, is of the type which has been used extensively for the past twenty years. Double walled tanks made of fiberglass reinforced resinous material are made by forming a second outer tank in two horizontal halves. The formed inner tank and outer tank halves are assembled and sealed to form a double walled storage tank system based on two rigid tanks with ribs therebetween.
A second method of making tanks from fiberglass reinforced resinous material utilizes a removable split half-mold with shapes for forming the support ribs and end cap along with the main body. After fiberglass and resinous material are applied to the mold and cured, the mold is removed. Next, the interior portions of the rib areas are filled with a filler material or bridged over with a cardboard insert and, fiberglass/resin applied so as to form a substantially smooth tank interior. A second tank half is formed in the same manner and joined with the first tank half. The formed tank is representative of those tanks wherein the support ribs are built into the inner tank as initially made. A double walled tank is made essentially by forming two additional horizontal tank halves, placing the initial single walled tank inside one horizontal half-tank and joining the second horizontal half-tank to the first.
U.S. Pat. No. 4,561,292, contains a description of another method of building a double walled storage tank. As is readily apparent from the patent and above discussion, building a double walled storage tank system with fiberglass and resin by known methods is very labor intensive and costly. There is also a concern about possible shifting of the support ribs away from the outer shell due to an inability to maintain close tolerances and inability to adequately hold the ribs to the outer shell. Recent concerns about leaked tanks has heightened the need for an efficient and economical manner of building a double walled storage tank system.
There has now been discovered a method whereby storage tanks are built with a double wall for secondary containment in an efficient, yet economical manner. Such tanks can also be equipped with means to monitor for any leakage.